Abstract
Self-regulation between structure and turbulence, which is a fundamental process in the complex system, has been widely regarded as one of the central issues in modern physics. A typical example of that in magnetically confined plasmas is the Low confinement mode to High confinement mode (L-H) transition, which is intensely studied for more than thirty years since it provides a confinement improvement necessary for the realization of the fusion reactor. An essential issue in the L-H transition physics is the mechanism of the abrupt “radial” electric field generation in toroidal plasmas. To date, several models for the L-H transition have been proposed but the systematic experimental validation is still challenging. Here we report the systematic and quantitative model validations of the radial electric field excitation mechanism for the first time, using a data set of the turbulence and the radial electric field having a high spatiotemporal resolution. Examining time derivative of Poisson’s equation, the sum of the loss-cone loss current and the neoclassical bulk viscosity current is found to behave as the experimentally observed radial current that excites the radial electric field within a few factors of magnitude.
Highlights
We perform systematic and quantitative validation study for the key physics for the radial electric field excitation in the L-H transition
The insert shows a schematic view of the measurement configuration, where two horizontal lines and four circles show the magnetic surfaces and the center of Heavy Ion Beam Probe (HIBP) measurement positions, respectively
Multipoint measurement of the electrostatic potential φ and the electron density ne is performed with a heavy ion beam probe (HIBP)
Summary
We perform systematic and quantitative validation study for the key physics for the radial electric field excitation in the L-H transition. Heavy Ion Beam Probe (HIBP) measurement was performed for direct observations of the radial electric field, the density gradient length, and the turbulent electrostatic potential fluctuation with a high spatiotemporal resolution. Taking into account the toroidal effect on the dielectric constant, we found that the sum of the loss-cone loss current and the neoclassical bulk viscosity current[10] meets the experimentally observed current that excites the radial electric field www.nature.com/scientificreports/. A small contribution of the turbulent Reynolds stress was confirmed. This is the first experimental study that quantitatively and systematically validates the theoretical models of the electric field bifurcation in the L-H transition
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